The present invention relates to a probe of a nuclear magnetic resonance apparatus, and in particular to a structure of a changeover switch.
In the nuclear magnetic resonance (NMR) apparatus, detected signals are feeble. In application such as the structure analysis of protein, it is demanded to have a higher sensitivity. The NMR apparatus has a higher resolution and a higher sensitivity as the static magnetic field strength becomes strong. A superconducting magnet for generating the static magnetic field tends to become large in size (“Present Status of 920 MHz High-Resolution NMR Spectrometers,” Kiyoshi et al, IEEE Trans. Appl. Supercond. Vol. 14, pp. 1608-1612 (2004)). As a result, a probe for disposing a detection coil called probe coil in a magnetic field center of the static magnetic field is also being made longer.
Since an NMR signal detected by the probe coil is very feeble, the NMR signal is typically amplified by a low noise pre-amplifier. The probe coil is connected to a low-noise pre-amplifier at a first stage by a transmission cable for high frequencies. The loss in this portion exerts a great influence on the sensitivity. The loss in the transmission cable becomes greater as the frequency becomes higher. In a high resolution NMR apparatus, therefore, the probe length becomes longer and the signal frequency becomes higher. As a result, the loss in the transmission cable tends to increase.
The simplest method to decrease the loss in a typical high frequency detection circuit is to shorten the transmission cable extended to the preamplifier at the first stage. However, this method is not easy in the case of the NMR probe. Since the probe coil conducts irradiation of a sample with an RF electromagnetic wave and detection of the NMR signal, a transmission/reception changeover switch is inserted in a stage preceding the preamplifier (JP-A-2002-207072).